Silicon carbide precursors and their preparation and use

ABSTRACT

(Dichloromethyl)methyldichlorosilane is reacted in the presence of Mg, K, Na or Li to form chloropolycarbosilanes which in turn can be reacted with a reducing agent to form polycarbosilane polymers that have utility as precursors to silicon carbide ceramics.

FIELD OF THE INVENTION

This invention relates to chloropolycarbosilanes, polycarbosilanes,their preparation, and their use for preparing silicon carbides.

BACKGROUND OF THE INVENTION

The most commonly used polycarbosilane precursor to silicon carbideceramics is "Nicalon", manufactured by the Nippon Carbon Company. (SeeS. Yajima, Am. Ceram. Soc. Bul., 62,893 (1982)). "Nicalon" is based onpolydimethylsilane [Si(CH₃)₂ ]_(n), a polymer which has a silicon tocarbon ratio (Si:C) of 1:2. In order to manufacture ceramic fibers froma ceramic precursor, it must retain its shape during firing. However, toaccomplish this with "Nicalon", firing must be carried out in thepresence of oxygen which stabilizes the "Nicalon" preceramic.

Silicon carbide ceramics are of growing technical and commercial importbecause of their use in high technology products. Thus, precursors tosilicon carbide and processes for their preparation offer fruitful areasfor research. In particular, novel highly crosslinked precursors forceramic fibers that lead to pure silicon carbide, SiC, and do not needoxygen curing are of great commercial interest.

SUMMARY OF THE INVENTION

A process is provided in accordance with this invention for preparing acrosslinked chloropolycarbosilane. The process comprises the step ofreacting Cl₂ Si(CH₃)CHCl₂ with a metal selected from the groupconsisting of Mg, K, Na, and Li in a solvent selected from the groupconsisting of toluene, xylene, and high boiling ethers at a temperaturefrom about 50° C. to reflux of said solvent. The invention also providesnovel chloropolycarbosilanes prepared by said process.

This invention further provides a process for preparing a crosslinkedpolycarbosilane which comprises the steps of preparing a crosslinkedchloropolycarbosilane as above, and reacting the chloropolycarbosilanewith a reducing agent at a temperature from -10° C. to 25° C. Theinvention also provides novel polycarbosilanes prepared by said process.

This invention further provides a process for preparing a siliconcarbide which comprises the steps of preparing a crosslinkedpolycarbosilane as above, and pyrolyzing said polycarbosilane.

Crosslinked chloropolycarbosilanes are provided in accordance with thisinvention which contain units of the formula ##STR1## wherein x, y, andz represent the proportion of bonds occupied by Cl, 0≦x<1, 0≦y<1, and0≦z<0.4.

Crosslinked polycarbosilanes are provided in accordance with thisinvention which contain units of the formula ##STR2## wherein x and yrepresent the proportion of bonds occupied by H, z represents theproportion of bonds occupied by Cl, 0≦x<1, 0≦y<1, and 0≦z<0.4.

DETAILED DESCRIPTION OF THE INVENTION

This invention provides for the reaction of Cl₂ Si(CH₃)CHCl₂ with Mg, K,Na, and Li in a suitable high boiling solvent to yield a crosslinkedchloropolycarbosilane. The solvent should boil at about 50° C. orhigher. Suitable solvents include toluene, xylene, and high boilingethers. Suitable ethers include cyclic and noncyclic compounds whichboil at about 50° C. or higher. The reaction is conducted at atemperature of from 50° C. to the solvent reflux temperature.

Reactions involving sodium can lead to degradation of various ethers(e.g., tetrahydrofuran) and incorporation of their byproducts into thepolymer product, and it is preferred to use toluene or xylene as thesolvent for reaction where Cl₂ Si(CH₃)CHCl₂ is reacted with Na.Preferred reaction systems involve refluxing Cl₂ Si(CH₃)CHCl₂ withmagnesium in tetrahydrofuran, and refluxing Cl₂ Si(CH₃)CHCl₂ with sodiumin toluene. In reactions involving Mg in tetrahydrofuran, some cleavageof the solvent may be observed (generally about 3%, or less) and thiscan result in the incorporation of minor amounts of oxygen in thechloropolycarbosilane product.

The form of the metal used is not considered crucial. Magnesium employedin the reaction may be standard reagent quality or "activated". Normallyfrom two to four moles of magnesium per mole of silane are employed.Lithium, sodium and potassium may be used in the form of ribbon, chunks,and/or dispersions. Normally at least about four moles of K, Na or Li isused per mole of silane.

The order of the addition, either silane to metal suspended in solventor the reverse, is not critical. In the case of metal added to silane,the addition is usually carried out over about 2 to 3 hours, but thistime is not critical. Metal addition is followed by a 2 to 3 hourreaction period at elevated temperatures--the reflux point of thesolvent is convenient--followed by, optionally, stirring at ambienttemperature for from 10 to 20 hours.

The crosslinked chloropolycarbosilane products of this process are noveland are useful as further described below for the production of siliconcarbide. For example, the process can be used to provide novel compoundscontaining units of the formula ##STR3## wherein x, y, and z representthe proportion of bonds occupied by Cl (the remainder representingcrosslinked bonds), 0≦x<1, 0≦y<1, and 0≦z<0.4. Typically, z is 0.2 orless. When Cl₂ Si(CH₃)CHCl₂ is essentially the only starting materialreacted with Mg, K, Na and/or Li, the chloropolycarbosilane may consistessentially of these units. It is noted that vinylchlorosilanes such as(CH₃)(CH₂ ═CH)SiCl₂ may be added to the reaction mixture to furtherenhance thermal crosslinking properties. In any case, the number averagemolecular weight of the chloropolycarbosilane (as calculated from thecorresponding polycarbosilane) typically ranges from about 300 to about17,000. When sodium is used as the active metal, x, y, and z are eachnormally about zero, and the product is typically an insoluble powderwhich can be directly pyrolyzed to silicon carbide.

The chloropolycarbosilanes prepared as above, particularly thosecontaining substantial amounts of chlorine bound to silicon (i.e.,Si--Cl functionality) can be reacted to form correspondingpolycarbosilanes using an appropriate reducing agent. For example, afterchloropolycarbosilane formation using magnesium, reduction may beaccomplished with lithium aluminum hydride (LAH). Other suitablereducing agents include lithium hydride and alkyl aluminum hydrides. Thereduction can conveniently be carried out in the same reaction vesselused for chloropolycarbosilane formation. The reducing agent is usuallyemployed in excess (from a stoichiometric amount to a 10% excess isnormal). The reduction temperature is usually from about -10° C. toabout room temperature (20°-25° C.). The preferred temperature is about0° C. Should the reduction reaction mass thicken, the reaction mass maybe thinned by the addition of an inert diluent such as hexane oradditional solvent. Typically, yellow glassy solids are obtained inyields from about 25-100%. The polymers are generally soluble in commonsolvents such as hexane, tetrahydrofuran and toluene.

The crosslinked polycarbosilane products of this process are novel andas further described below, are precursors to silicon carbide. Forexample, the process can be used to provide novel compounds containingunits of the formula ##STR4## wherein x and y represent the proportionof bonds occupied by H, z represents the proportion of bonds occupied byCl, 0≦x<1, 0≦y<1, and 0≦z<0.4. Typically, z is 0.2 or less.

Typically (as shown by Silicon-29 NMR analysis) most of the silicon inthe crosslinked polycarbosilane is surrounded by four carbon atoms(i.e., x and y are low). However, some SiH and SiH₂ are normallyobserved when magnesium is used as the metal for forming thechloropolycarbosilane. For example, when magnesium is used, the siliconin the polycarbosilane is typically present as about 14% SiH, about 5%SiH₂ and about 81% with no hydrogen attached. GPC analysis of thesepolycarbosilanes normally shows a broad bimodal distribution (e.g.,Mn=1700, Mw=9500 with a polydispersity of 5.5). These polymers typicallyhave a number average molecular weight within the range from about 250to 17,000, and contain about 7-8% by weight residual chlorine. Typicallythese polymers do not melt below 360° C.; but DSC analysis of thepolymer under nitrogen gives two exothermic transitions: at about240°-265° C. and at about 370°-385° C. In air, a third exothermictransition is observed at about 449° C.

Generally the reaction of Cl₂ Si(CH₃)CHCl₂ with magnesium in accordancewith this invention, and subsequent reduction of the reaction productyields a highly crosslinked polycarbosilane which remains soluble. It isconsidered unusual to obtain high molecular weight material usingmagnesium metal and production of higher weight materials in accordancewith this invention may be due to crosslinking at the two reactive CHCl₂sites. It is noted that the chloropolycarbosilane may also be reactedwith water (rather than a reducing agent) to give uniquepolyoxocarbosilane products which are precursors of silicon oxycarbides.

The polycarbosilanes obtained by reducing chloropolycarbosilanes havingsubstantial Si--Cl functionality, can be pyrolyzed under argon toproduct black ceramics. The following pyrolysis condition can beemployed. For pyrolysis at 1000° C., increase the temperature from roomtemperature (i.e. about 25° C.) to 1000° C. at 5° C./min, hold thetemperature at 1000° C. for about 1 hour, and cool to room temperature(e.g. at 5° C./hr). Weight loss normally occurs in two steps during theheating process, between about 150° C. and 300° C., and between about400° C. and 550° C. It is preferred to avoid oxygen curing of thepolycarbosilane.

When magnesium is used as the metal for preparing thechloropolycarbosilane intermediates, ceramic yields upon the pyrolysisof the polycarbosilanes derived therefrom, typically range from about 55to 63% (TGA analysis from 25° C. to 950° C. with a 10° C./min ramp hasindicated similar yields). Curing of the polycarbosilanes under argonduring the pyrolysis process (e.g. at 200° C., 250° C. or 370° C.) canincrease the ceramic yield slightly. Fibers of these polycarbosilanes(i.e., where magnesium is used to prepare the chloropolycarbosilaneintermediate) can be pulled from hexane or toluene solution. Normallysuch fibers retain shape when pyrolyzed under argon to 1000° C. (withoutoxygen curing).

When sodium is used as the metal for preparing the intermediatepolymers, an insoluble polycarbosilane is normally produced. Thesepolymers can be characterized as highly crosslinked mixedpolysilane/polycarbosilane systems. Ceramic yields upon pyrolysis ofthese polymers typically range from about 68 to 79%. The use ofpotassium as the metal for preparing the intermediate polymers isconsidered to provide silimar results. When lithium is used as the metalfor producing the intermediate polmers a yellow liquid polymer isgenerally obtained. Pyrolysis of this polymer under argon typicallyprovides a ceramic yield of about 40%.

Practice of the invention will become further apparent from thefollowing non-limiting Examples.

EXAMPLES

Reactions were carried out in dried equipment under an argon atmosphere.Impure (dichloromethyl)methyldichlorosilane (i.e., Cl₂ Si(CH₃)CHCl₂) wasobtained from a Du Pont manufacturing process and purified bydistillation. Toluene was reagent grade and purified by distillationfrom lithium aluminum hydride before use. Tetrahydrofuran wasspectrophotometric grade (EM Science) and purified by distillation fromsodium/benzophenone before use. The magnesium employed was obtained fromAldrich as "99.5% activated chips". Hexane was spectrophotometric gradefrom EM Science. Lithium aluminum hydride was "95+%" from Aldrich.Sodium was obtained from J. T. Baker, Inc. and cut into roughly 1 cmcubes before use. Sodium dispersion was obtained from Aldrich (40% byweight in mineral spirits). Proton NMR spectra were determined indeuterochloroform solvent on a GE model QE-300 instrument. Silicon-29NMR were determined in deuterochloroform solvent on a GE model omega 300instrument. Thermogravimetric analyses were performed on a Du Pont Model951 analyzer. Elemental analyses of the ceramics were performed byCorning Incorporated.

EXAMPLE 1 Preparation of Chloropolycarbosilane

To a stirred solution of 43.0 ml (0.307 mol) of Cl₂ MeSiCHCl₂ in 400 mlTHF was added 15.62 g (0.643 mol) of dried magnesium turnings over a 1hr period at 25° C. After the addition, the reaction was refluxed for 2hr, and the resulting orange-brown mixture was cooled and stirred atroom temperature for 16 hr.

EXAMPLE 2 Preparation of Polycarbosilane

The mixture obtained in Example 1 was cooled to 0° C. and 3.20 g oflithium aluminum hydride was added slowly. After stirring for 1 hr, theslurry was slowly quenched with 10% aqueous hydrochloric acid solutionuntil the evolution of gas ceased. The bulk of the solids were filteredand the filtrate was extracted with hexane and washed once withsaturated ammonium chloride solution, then twice with distilled waterand dried over magnesium sulfate. Removal of the volatiles in vacuo left17.8 g (104% based on (MeSiCH)_(n)) of a golden brown solid. The polymeris soluble in most common organic solvents. Analysis: Calcd for C₂ H₄.5SiCl₀.11 : C, 39.68; H, 7.49; Cl, 6.44. Found: C, 38.53; H, 7.30; Cl,6.20. Molecular weight analysis (GPC polystyrene standard) of thepolycarbosilane obtained from a similar preparation gave Mn=1700,Mw=9500, D=5.5

EXAMPLE 3 Preparation of Silicon Carbide

Pyrolysis of the solid polycarbosilane obtained in Example 2 from 25° C.to 1000° C. at 5° C./min with a 1 hr hold at 1000° C. gave a blackceramic in 56% yield. Pyrolysis of polycarbosilanes obtained in similarpreparations under the same conditions gave shiny-black ceramics withyields ranging from about 55-63%. Analysis of these ceramics, obtainedfrom polycarbosilanes similar to that in Example 2, gave ranges ofSiC+0.44-0.50 C+0.21-0.26 O. Drawn fibers of the ceramic precursorsusing hexane as a solvent retained their shape during firing in an argonatmosphere

EXAMPLE 4 Preparation of Polysilane/Polycarbosilane

To a stirred mixture of 5.29 g (0.230 mol) of sodium chunks in 150 mL oftoluene was added 7.10 mL (0.0506 mol) of Cl₂ MeSiCHCl₂ dropwise over aperiod of 1 hr. The reaction was refluxed for 3 hr and the resultingpurple solution was cooled to room temperature. After stirring for 16hr, the reaction mixture and excess sodium metal were quenched withisopropanol followed by distilled water until evolution of gas ceased.The solution was washed with saturated ammonium chloride solution.Isopropanol was added to precipitate 1.04 g (37%) ofpolysilane/polycarbosilane as a off-white powder. The powder wasinsoluble after precipitation. Isolation of insoluble solids from theaqueous washings afforded another 0.428 g (15%) of the mixedpolysilane/polycarbosilane polymer. Analysis: Calculated for[(CH₃)SiCH]_(n), C₂ H₄ Si: C, 42.79; H, 7.18. Found: C, 39.95; H, 6.75(insoluble polymer). Found C, 44.57; H, 6.99 (precipitated powder).

EXAMPLE 5 Preparation of Silicon Carbide

Pyrolysis of the precipitated powder from Example 4 from 25° C. to 1000°C. at 5° C./min with a 1 hour hold at 1000° C. gave a black ceramic inabout 79% yield. Pyrolysis of the residual insoluble polymer fromaqueous washings in Example 4 under the same conditions have a blackceramic in about 68% yield.

EXAMPLE 6 Preparation of Chloropolycarbosilane

A solution containing 19.56 mL (0.140 mol) of Cl₂ SiCHCl₂ and 2.02 mL(0.015 mol) of (CH₃)(CH₂ ═CH)SiCl₂ were added dropwise over a 1 hrperiod to a stirred mixture of 8.31 g (0.342 mol) of magnesium metal in150 mL of tetrahydrofuran. After the addition the brown mixture wasrefluxed for 16 hr and cooled to room temperature.

EXAMPLE 7 Preparation of Polycarbosilane

The mixture obtained in Example 6 was cooled to 0° C. and 4.86 g (0.128g) of lithium aluminum hydride was slowly added. After stirring for 1hr, the slurry was refluxed for 30 min, cooled and was slowly quenchedwith 10% aqueous hydrochloric acid solution until the evolution of gasceased. The bulk of the solids were filtered and the filtrate wasextracted with hexane and washed once with saturated ammomium chloridesolution, then twice with distilled water and dried over magnesiumsulfate. Removal of the solvent in vacuo left 2.26 g (29%) of a goldenbrown liquid. Proton NMR showed the polymer to be {[(CH₃)SiCH(H)₁.6]₀.95 [(CH₃)(CH₂ ═CH)SiH]₀.05 }. Analysis: Calculated for C₂.0 H₅.7SiCl₀.4 ; C, 40.91; H, 8.96; Cl, 2.29. Found: C, 39.29; H, 8.60; Cl,2.21. TGA yield (25°-1000° C. at 10°/min)=46%.

EXAMPLE 8 Preparation of Polyoxocarbosilane

To the orange brown mixture synthesized similar to that in Example 1from 42.0 mL (0.300 mol) of Cl₂ MeSiCHCl₂, 14.92 g (0.614 mol) ofmagnesium chips in 500 mL of tetrahydrofuran was added excess distilledwater to quench the residual Si--Cl functionality. The polymer wasextracted with hexane and washed once with saturated sodium bicarbonatesolution to remove excess hydrochloric acid, twice with distilled waterand dried over magnesium sulfate. Removal of the volatiles in vacuo left17.5 g (90%) of a thick viscous yellow polymer of the formula[(CH₃)SiCHCl₀.056 O₀.44 ]_(n). Analysis: Calculated for C₂ H₄ SiCl₀.056O₀.44 : C, 36.87; H, 6.19; Cl, 3.05. Found: C, 36.91; H, 6.18; Cl, 3.05.TGA=69% ceramic yield.

What is claimed is:
 1. A process for preparing a crosslinkedchloropolycarosilane comprising the step of: reacting Cl₂ Si(CH₃)CHCl₂with a metal selected from the group consisting of Mg, K, Na, and Li ina solvent selected from the group consisting of cyclic and non-cyclicethers which boil at about 50° C. or higher, toluene and xylene, at atemperature from about 50° C. to reflux of said solvent.
 2. The processof claim 1 wherein the crosslinked chloropolycarbosilane contains unitsof the formula ##STR5## wherein x, y, and z represent the proportion ofbonds occupied by Cl, 0≦x<1, 0≦y<1, and 0≦z<0.4.
 3. The process of claim2 wherein the metal is Mg and the solvent is tetrahydrofuran.
 4. Theprocess of claim 2 wherein the metal is Na, the solvent is toluene, andx, y, and z are each about zero.
 5. A crosslinked chloropolycarbosilaneprepared by the process of claim
 1. 6. A crosslinkedchloropolycarbosilane containing units of the formula ##STR6## whereinx, y, and z represent the proportion of bonds occupied by Cl, 0≦x<1,0≦y<1, and 0≦z<0.4.
 7. A process for preparing a crosslinkedpolycarbosilane which comprises the steps of:preparing a crosslinkedchloropolycarbosilane by the process of claim 1; and reducing saidchloropolycarbosilane at a temperature from <10° C. to 25° C.
 8. Theprocess of claim 7 wherein the crosslinked chloropolycarbosilanecontains units of the formula ##STR7## wherein x, y, and z represent theproportion of bonds occupied by Cl, 0≦x<1, 0≦y<1, and 0≦z<0.4; andwherein the crosslinked polycarbosilane has the formula ##STR8## whereinx and y represent the proportion of bonds occupied by H, z representsthe proportion of bonds occupied by Cl, 0≦x<1, 0≦y<1, and 0≦z<0.4. 9.The process of claim 7 wherein the metal is Mg and the solvent istetrahydrofuran.
 10. A crosslinked polycarbosilane prepared by theprocess of claim
 7. 11. A crosslinked polycarbosilane containing unitsof the formula ##STR9## wherein x and Y represent the proportion ofbonds occupied by H, z represents the proportion of bonds occupied byCl, 0≦x<1, 0≦y<1, and 0≦z<0.4.
 12. A process for preparing a siliconcarbide which comprises the steps of: preparing a crosslinkedpolycarbosilane by the process of claim 7; and pyrolyzing saidpolycarbosilane.
 13. The process of claim 12 wherein the crosslinkedchloropolycarbosilane contains units of the formula ##STR10## wherein x,y, and z represent the proportion of bonds occupied by Cl, 0≦x<1, 0≦y<1,and 0≦z<0.4; and wherein the crosslinked polycarbosilane has the formula##STR11## wherein x and y represent the proportion of bonds occupied byH, z represents the proportion of bonds occupied by Cl, 0≦x<1, 0≦y<1,and 0≦z<0.4.
 14. The process of claim 13 wherein the metal is Mg and thesolvent is tetrahydrofuran.